In cellular radiotelephone communication systems, call handoffs are often required in order to maintain communication with mobiles as they move within the system. Handoff is the process whereby an in progress call is transferred from one RF coverage area (cell) to another in coordination with a mobile's movements. It is also the process whereby a call is transferred to another channel within a cell because of interference within the cell. The process of handing off an in progress call is one of the most delicately balanced functions relating to cellular radiotelephone systems and service because of the high level of coordination required among various system processing elements. Thus, failure to hand a call off at the proper time may result in problems like, reduced call quality, interference with neighboring coverage areas and even the undesired termination of the call.
In order to effectuate successful handoffs, current analog cellular systems continuously monitor the quality of every call which is operational on the system. The system must recognize when the quality of a call falls below a predetermined threshold in a particular cell and must also determine what other neighboring cell can satisfactorily handle the call. Once a more suitable candidate is identified, the system sends instructions to the mobile directing it to tune to another channel. The mobile confirms that it is leaving its current channel, tunes to the new channel, synchronizes to the new channel and begins transmitting.
As will be appreciated, in analog cellular systems, handoff processing is dependant on the state transitions of various supervisory signals generated by the mobile unit. Unfortunately, interfering mobile units can falsify these signals, thereby causing poor call quality during handoffs, or in some cases causing a calls termination.
In digital cellular radiotelephone systems, the procedure is modified somewhat in that the mobile measures channels in other cells as well as its current serving channel and reports these measurements, consisting of signal strength back to the system. Since, the mobile cannot determine whether its measurements are as a result of interference, the system must in addition, scan for the mobile in the cell selected by the mobile. Unfortunately, the system scan occurs after the quality of the in progress call has fallen below a predetermined threshold. Thus, while mobile assisted handoff tends to reduce the number of candidate target cells, no real improvement in call quality during the handoff is achieved.
It would be extremely advantageous therefore to provide a handoff procedure which is less susceptible to interference and provides some degree of fault tolerance.